The semicircular canals (SC) in the inner ear can develop various defects. For example, a dehiscence, e.g., a superior canal dehiscence (SCD), is a bony defect of the canal, e.g., the superior semicircular canal (SSC) that, when associated with vestibular and auditory complaints, is referred to as SCD syndrome (SCDS). Signs and symptoms of SCDS are believed to arise from a so-called “Third Window” phenomenon. Sound induced movement at the stapes footplate normally results in fluid movement through the high impedance cochlea. However, in SCD, a third window at the SSC results in shunting of inner ear fluids through the membranous labyrinth, towards this area of lower impedance. This results in auditory induced vestibular symptoms such as dizziness triggered by sound (Tullio's sign) or pressure (Hennebert's sign). Such dehiscence can also occur in the lateral and posterior semicircular canals.
A reduction in the pressure across the cochlear partition and an increase in bone-conducted, cochlear-evoked potentials lead to either hearing loss (seen as a low frequency air bone gap) or conductive hyperacusis (with “supra-normal bone conduction thresholds seen at −5 or −10 dB). Classic symptoms include aural fullness, pulsatile tinnitus, hyperacusis, and autophony, in addition to dizziness and vertigo. Surgical intervention is reserved only for patients with debilitating auditory and/or vestibular symptoms, because, as currently performed, this procedure carries a risk of partial or complete hearing loss in the affected ear, as well as persistent disabling dizziness or vertigo.
Several types of SCD repair have been described, and fall into two general categories: “resurfacing procedures” and “occluding procedures.” Resurfacing is performed by placement of a bone chip, fascia, cartilage, bone cement, or another material directly over the defect, which effectively places a “cap” over the bony defect and attempts to “close” the third window without affecting the membranous inner ear. Occluding techniques are performed by placement of bone wax, bone chips, or fascia within the lumen of the canal to destroy the membranous inner ear and occlude both limbs of the canal, which effectively “plugs” the defect.